1. Field of the Invention
The present invention relates to a fluid ejection device with staggered ink drop generators.
2. Related Art
Thermal ink jet (TIJ) printers are popular and widely used in the computer field. These printers are described by W. J. Lloyd and H. T. Taub in “Ink Jet Devices,” Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Ink jet printers produce high-quality print, are compact and portable, and print quickly and quietly because only ink strikes a print medium (such as paper).
An ink jet printer produces a printed image by printing a pattern of individual dots (or pixels) at specific defined locations of an array. These dot locations, which are conveniently visualized as being small dots in a rectilinear array, are defined by the pattern being printed. The printing operation, therefore, can be pictured as the filling of a pattern of dot locations with dots of ink.
Ink jet printers print dots by ejecting a small volume of ink onto the print medium. An ink supply device, such as an ink reservoir, supplies ink to the ink drop generators. The ink drop generators are controlled by a microprocessor or other controller and eject ink drops at appropriate times upon command by the microprocessor. The timing of ink drop ejections generally corresponds to the pixel pattern of the image being printed.
In general, the ink drop generators eject ink drops through an orifice (such as a nozzle) by rapidly heating a small volume of ink located within a vaporization or firing chamber. The vaporization of the ink drops typically is accomplished using an electric heater, such as a small thin-film (or firing) resistor. Ejection of an ink drop is achieved by passing an electric current through a selected firing resistor to superheat a thin layer of ink located within a selected firing chamber. This superheating causes an explosive vaporization of the thin layer of ink and an ink drop ejection through an associated nozzle of the printhead.
Ink drop ejections are positioned on the print medium by a moving carriage assembly that supports a printhead assembly containing the ink drop generators. The carriage assembly traverses over the print medium surface and positions the printhead assembly depending on the pattern being printed. The carriage assembly imparts relative motion between the printhead assembly and the print medium along a “scan axis”. In general, the scan axis is in a direction parallel to the width of the print medium and a single “scan” of the carriage assembly means that the carriage assembly displaces the printhead assembly once across approximately the width of the print medium. Between scans, the print medium is typically advanced relative to the printhead along a “media advance axis” that is perpendicular to the scan axis (and generally along the length of the print medium).
As the printhead assembly is moved along the scan axis a swath of intermittent lines are generated. The superposition of these intermittent lines creates the appearance as text or image of a printed image. Print resolution along the media advance axis is often referred to as a density of these intermittent lines along the media advance axis. Thus, the higher the density of the intermittent lines in the media advance axis the greater the print resolution along that axis.
The density of the intermittent lines along the media advance axis (and thus the paper axis print resolution) can be increased by adjusting the “step” between sequential scans. For example, if it takes an average of two steps to cover a swath equal to the length of a nozzle array aligned with the media advance axis, this is referred to as “two-pass printing”. The swaths in this case would be offset by a distance equal to a non-integer number of nozzle pitch lengths (measured along paper axis) to allow the pitch of intermittent lines to be halved. This effectively doubles the resolution along the paper axis. One major disadvantage, however, of two-pass printing is that the extra passes greatly decrease the speed of the printer. For instance, two-pass printing is about half the print speed of one-pass printing. Such a large decrease in print speed is undesirable for some printing operations, but acceptable in others.
Another technique that may be used to increase the density of the intermittent lines along the media advance axis is to increase the density of the nozzle spacing to provide a high print resolution in one-pass printing. However, it is quite difficult to manufacture ink drop generator and nozzle structures that allow the high linear density of nozzles required for high print resolution printing. For instance, ink drop generators must be fine enough to allow for tight spacing, ink drop volume must decrease with the tighter spacing, and the subsequent lower drop volume may not be compatible with the desired print mode. There exists a need, therefore, for an ink jet printhead capable of multi-mode operation that allows for high-resolution, high-speed printing in one print application while also providing a high resolution maximum quality print mode in another print application.